The present invention relates to a power converter or power controlling system for controlling an output voltage by pulse width modulation. The power converter can be used as inverter, converter or chopper in a variable speed drive system, an uninterruptible power system (UPS) and a reactive power compensating system (SVC).
For example, a conventional inverter for driving a three phase ac motor is arranged to switch a main circuit component such as a bipolar transistor or an IGBT, and to output a PWM voltage. FIG. 12 shows one conventional example.
The three-phase inverter of FIG. 12 includes a converter section 1, an inverter main section 2 and an ac motor 3 driven by the inverter, and a control circuit 4 for controlling the inverter. The inverter main section 2 includes switching devices S1.about.S6. The control circuit 4 produces gate signals GS1.about.GS6 for the switching devices S1.about.S6. With these gate signals GS1.about.GS6, the inverter can control the average voltage by controlling the on/off duty ratio.
As main circuit element, the inverter employs bipolar transistor or IGBT in a capacity region under a few hundred KVA, and GTO beyond that region.
The high speed switching operation of a high speed switching device such as IGBT causes the output voltage to vary at high speeds. As a result, a high frequency component in the output voltage waveform is delivered as noise, and causes malfunction and failure in associated devices by radiation of radio waves, undesired magnetic coupling, or superimposition on a power line.
Therefore, a conventional inverter as shown in FIG. 13 employs one or more noise filters composed of passive components such as LCR, at input or output terminals or in control signal lines to reduce leakage of the high frequency component to the outside.